This invention relates to flame-retardant thermoplastic compositions having improved dielectric properties and a process for the preparation thereof. More particularly, it pertains to compositions comprising a high molecular weight linear polyester, a flame-retardant, a particulate material such as mica or clay, and, optionally, a reinforcing agent and a process for improving the arc track resistance of reinforced, flame-retardant linear high molecular weight polyester compositions without impairing the flame-retardancy and other desirable properties such as strength, modulus and heat deflection temperature.
High molecular weight linear polyesters and copolymers of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al., U.S. Pat. No. 2,465,319 and in Pengilly, U.S. Pat. No. 3,047,539. These patents disclose that the polyesters are particularly advantageous as film and fiber-formers.
With the development of molecular weight control, use of nucleating agents and two-step molding cycles, poly(ethylene terephthalate) has become an important constituent of injection moldable compositions. Poly(1,4-butylene terephthalate), because of its very rapid crystallization from the melt, is uniquely useful as a component in such compositions. Workpieces molded from such polyester resins, in comparison with other thermoplastics, offer a high degree of surface hardness and abrasion resistance, high gloss, and lower surface friction.
One useful family of such compositions comprises those which are reinforced, e.g., with from about 10 to about 40% of filamentous glass, based on the weight of glass and polyester components, and rendered flame-retardant by the incorporation of a flame-retarding amount of a flame-retarding component.
Another useful family of such compositions comprises those containing poly(1,4-butylene terephthalate), a polycarbonate resin, a flame-retardant and, optionally, a reinforcing agent.
Experience has shown, however, that articles molded from these afore-described compositions are characterized with poor electrical arc resistance properties.
It has now been surprisingly discovered that the incorporation of a particulate material such as finely divided mica or finely divided surface treated clay improves the high voltage arc track resistance or reinforced or unreinforced flame-retarded polyester resins while not adversely affecting the flame-retardancy and the other excellent physical properties of the composition.